MIC27600 datasheet(13/27 Pages) MICREL

13 / 27 page

Micrel, Inc.

MIC27600

October 2010

M9999-101310

Functional Description

The MIC27600 is an adaptive ON-time synchronous

step-down DC-DC regulator. It is designed to operate

over a wide input voltage range from, 4.5V to 36V, and

provides a regulated output voltage at up to 7A of output

current. A digitally modified adaptive ON-time control

scheme is employed in to obtain a constant switching

frequency and to simplify the control compensation.

Over-current protection is implemented without the use

of an external sense resistor. The device includes an

internal soft-start function which reduces the power

supply input surge current at start-up by controlling the

output voltage rise time.

Theory of Operation

Figure 1 illustrates the block diagram for the control loop

of the MIC27600. The output voltage is sensed by the

MIC27600 feedback pin FB via the voltage divider R1

and R2, and compared to a 0.8V reference voltage VREF

the

error

comparator

through

low

gain

transconductance (gm) amplifier. If the feedback voltage

decreases and the output of the gm amplifier is below

0.8V, then the error comparator will trigger the control

logic and generate an ON-time period. The ON-time

period length is predetermined by the “FIXED tON

ESTIMATION” circuitry:

300kHz

OUT

ed)

ON(estimat

Eq. 1

where VOUT is the output voltage and VIN is the power

stage input voltage.

At the end of the ON-time period, the internal high-side

driver turns off the high-side MOSFET and the low-side

driver turns on the low-side MOSFET. The OFF-time

period length depends upon the feedback voltage in

most cases. When the feedback voltage decreases and

the output of the gm amplifier is below 0.8V, the ON-time

period is triggered and the OFF-time period ends. If the

OFF-time period determined by the feedback voltage is

less than the minimum OFF-time tOFF(min), which is about

360ns, then the MIC27600 control logic will apply the

tOFF(min) instead. tOFF(min) is required to maintain enough

energy in the boost capacitor (CBST) to drive the high-

side MOSFET. The maximum duty cycle is obtained

from the 360ns tOFF(min):

OFF(min)

max

360ns

−

Eq. 2

where tS = 1/300kHz = 3.33μs. It is not recommended to

use MIC27600 with a OFF-time close to tOFF(min) during

steady-state operation. Also, as VOUT increases, the

internal ripple injection will increase and reduce the line

regulation performance. Therefore, the maximum output

voltage of the MIC27600 should be limited to 5.5V.

Please refer to “Setting Output Voltage” subsection in

Application Information

for more details.

The actual ON-time and resulting switching frequency

will vary with the part-to-part variation in the rise and fall

times of the internal MOSFETs, the output load current,

and variations in the VDD voltage. Also, the minimum tON

results in a lower switching frequency in high VIN to VOUT

applications, such as 26V to 1.0V. The minimum tON

measured on the MIC27600 evaluation board is about

184ns. During load transients, the switching frequency is

changed due to the varying OFF-time.

To illustrate, the control loop operation will be analyzed

in both steady-state and load transient scenarios. For

easy analysis, the gain of the gm amplifier is assumed to

be 1. With this assumption, the inverting input of the

error comparator is the same as the feedback voltage.

Figure 2 shows the MIC27600 control loop timing during

steady-state operation. During steady-state, the gm

amplifier senses the feedback voltage ripple, which is

proportional to the output voltage ripple and the inductor

current ripple, to trigger the ON-time period. The ON-

time is predetermined by the tON estimator. The

termination of the OFF-time is controlled by the feedback

voltage. At the valley of the feedback voltage ripple,

which occurs when VFB falls below VREF, the OFF period

ends and the next ON-time period is triggered through

the control logic circuitry.

Figure 2. MIC27600 Control Loop Timing

MIC27600 Datasheet(PDF) 13 Page - Micrel Semiconductor

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